Among surface treated steel sheets, a black steel sheet is a colored steel sheet with an inorganic black coating formed on a surface thereof through a blackening treatment. Since the black steel sheet enables customers to remove a painting process, it may reduce manufacturing costs, and also, since the black surface appearance thereof is uniform and reasonably attractive, the black steel sheet is widely used in various fields, such as home appliance, audio devices, OA devices, and vehicle components. A blackening treatment of a zinc plated steel sheet is mainly performed by an etching process, a cathode electrolysis process, an anode electrolysis process, or the like, and an inorganic blackened coating has microcracks or micropores formed in a surface thereof and is in a chemical form, such as an inorganic compound (e.g., metal oxide, metal hydroxide, or metal), particularly, in the form of metal oxide. The mechanism by which the inorganic blackened coating is tinged with black is explained by irregular reflection of incident light due to micropores and the absorption characteristics of visible light, depending on a metal oxide.
Such a black steel sheet has been manufactured by a method of forming a black coating through oxidation, an anode treatment or a conversion treatment with a zinc alloy plated steel sheet, mainly a Zn—Ni alloy plated steel sheet.
As a representative example, patent documents 1 and 2 disclose a method of forming a black coating on a zinc alloy plated steel sheet through oxidation with an acid aqueous solution containing metal ions, such as Ni, Co, Fe, Al, Mg, Cu, Sn, C, Cr, Mo, Ag, or the like. Also, patent documents 3 and 4 disclose a method of forming a black coating through an anode treatment of a general steel sheet or a surface treated steel sheet in an aqueous solution, and patent documents 5 and 6 disclose a method of forming a black coating through a conversion treatment of zinc or zinc alloy plated treated steel sheet in a solution containing metal ions.
Until the 1990s, blackening treatment methods using anode electrolysis, cathode electrolysis, conversion treatments, and the like were mainly developed, but in recent years, technological developments have been directed toward targets to add or enhance physical properties (e.g., processability, corrosion resistance, surface appearance, etc.). Patent documents 7 and 8 explain heat absorption and emission properties, conductance, electromagnetic wave shielding properties, and the like of black steel sheets which are subject to a blackening treatment and mainly use a Zn—Ni plated steel sheet having excellent blackened film adhesiveness as a base steel sheet. However, these related arts have a limitation, such as a rise in processing costs, as the black coating is formed by using an electrolytic process, such as an anode electrolytic process or a cathode electrolytic process, and also has a limitation that the occurrence of a serious powdering phenomenon in which the blackened film is broken or detached due to a lowering of adhesive force in spite of the conversion treatment of a zinc plated steel sheet.
Additionally, patent document 9 discloses a method of forming a black film on a zinc plated or a zinc alloy plated steel sheet by using a solution containing Sn and an Ni or Co compound, but in the case of the blackened film formed by this method, powdering is serious and thus adhesiveness of the blackened film is lowered. Also, in the case of a conversion treatment as above, since the reaction rate for forming the blackened film is generally slower than that in the electrolytic process, the conversion treatment is not suitable for working in combination with the electrical plating line operating in a high speed/continuous process and is also low in terms of productivity, compared with the electrolytic process. Further, patent document 10 discloses a method of blackening a steel sheet by reforming a surface film in a high temperature and high humidity atmosphere, but has a limitation in that the method is a continuous process needing a few tens of minutes of treatment time.
The blackened film may be formed by oxidizing a surface metal film by using a cathode electrolysis, an anode electrolysis, an oxidation, a conversion treatment, or the like, or substitution-precipitating a metal different from a base steel sheet. Since the foregoing methods lower corrosion resistance of the blackened film, a chromate treatment is performed to overcome such a limitation, but the enforcement of Cr control needs an alternative to such a control, and thus a conversion coating film suitable for a blackened film different from a conventional Cr-free blackened film is required.